Muwatta Malik, Book 8, Hadith 325 (In-Book)

Yahya related to me from Malik that he had heard that A'isha, the wife of the Prophet, may Allah bless him and grant him peace, used to pray in a shift and head-covering.

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ عَائِشَةَ، زَوْجَ النَّبِيِّ صلى الله عليه وسلم كَانَتْ تُصَلِّي فِي الدِّرْعِ وَالْخِمَارِ ‏.‏

sunnah.com

also referenced under: Book 8, Hadith 36 (USC-MSA)